1. Field of the Invention
The present invention relates to a parametric amplifier; and, more particularly, to a parametric amplifier having a single varactor diode and a broadband termination of the upper sideband frequency.
2. Description of the Prior Art
Parametric amplification refers to a class of amplifiers where amplification at microwave frequencies has been achieved, wherein the output power is derived from an A.C. input, and derives its name from the fact that the differential equation governing the circuit behavior contains one or more reactive parameters which are non-linear or time varying. Parametric amplifiers can operate on the basis of periodically varying the capacitance of a varactor diode by means of a high frequency pump signal and transferring energy to a lower frequency microwave input signal by mixing of the input frequency with the much higher pump frequency through an idler frequency. The Manley-Rowe relations show that the current must flow at the three frequencies satisfying the relation:
Pump Frequency= Signal Frequency+ Idler Frequency
for gain to exist at the signal frequency.
A parametric amplifier employs one or more varactor diodes and a pump frequency source to obtain gain with extremely low noise figures. The pump frequency source strongly modulates or pumps the capacitance of the varactor diode at the pump frequency rate. Sum (upper sideband) and difference (idle) frequencies are generated due to the nonlinearity of the varactor diodes. The idle frequency circuit is reactively tuned (no resistive loading) for maximum current flow at the idle frequency while the upper sideband frequency circuit is adjusted for zero current flow at that frequency to achieve maximum gain and low noise figure characteristics.
Obtaining operation with efficient pump coupling to the varactor diode and minimal interaction between the upper sideband, idle, signal, and pump frequencies has been difficult to achieve in previous types of parametric amplifiers which generally have uncontrolled frequency terminations. Consequently, complex multiple tuning designs have been required to manually tune each parametric amplifier but, in general, these tuning designs do not result in optimum broadband performance. For example, one such attempt to control the upper sideband frequency impedance for a balanced diode amplifier is described in U.S. Pat. No. 3,609,571, issued Sept. 28, 1971, "Sideband Suppression for Broadband Parametric Amplifier", Klein et al. With this arrangement, tuning screws were required which did not provide truly independent adjustment of the pump and the upper sideband frequencies causing the parametric amplifier adjustments to be extremely critical with resulting high production costs and high pump power.
U.S. Pat. No. 3,992,675, issued Nov. 16, 1976, Niehenke and Linsenbardt, provides a parametric amplifier having a balanced varactor diode arrangement to parametrically couple the pump and input signals and a resonating waveguide with upper and lower choke waveguide combined with a terminating waveguide. The upper sideband signal current is inhibited by having the terminating waveguide present a reactance to the diodes that is the conjugate of the reactance presented by the upper and lower choke waveguide resulting in an open-circuit termination at the upper sideband frequency.
U.S. Pat. No. 3,518,562, issued June 30, 1970, Lombardo et al., provides a parametric amplifier with a single varactor diode and an adjustable filter device in the signal transmission line. U.S. Pat. No. 3,805,176, issued Apr. 16, 1974, Tatsuo Kudo, et al., provides a single varactor diode parametric amplifier with a choke filter to choke the pump frequency power and another choke filter to choke the idler frequency power, both chokes being in the signal transmission line. No provision is made for inhibition of the upper sideband signal frequency in the input signal transmission line.
It is therefore desirable to provide a parametric amplifier having a single varactor diode with a low noise figure, high gain, and low power consumption.
It is therefore desirable to provide a parametric amplifier with a single varactor diode and a broadband termination at the upper sideband frequency incorporated in the pump circuit path.
It is also desirable to provide a parametric amplifier with single varactor diode and an upper sideband frequency choke incorporated in the signal circuit path to prevent the upper sideband frequency signals from going to the signal output.